bpf: Add tests for bpf_bprm_opts_set helper

The test forks a child process, updates the local storage to set/unset
the securexec bit.

The BPF program in the test attaches to bprm_creds_for_exec which checks
the local storage of the current task to set the secureexec bit on the
binary parameters (bprm).

The child then execs a bash command with the environment variable
TMPDIR set in the envp.  The bash command returns a different exit code
based on its observed value of the TMPDIR variable.

Since TMPDIR is one of the variables that is ignored by the dynamic
loader when the secureexec bit is set, one should expect the
child execution to not see this value when the secureexec bit is set.
Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20201117232929.2156341-2-kpsingh@chromium.org

tools/testing/selftests/bpf/prog_tests/test_bprm_opts.c

+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright (C) 2020 Google LLC.
+ */
+
+#include <test_progs.h>
+#include <linux/limits.h>
+
+#include "bprm_opts.skel.h"
+#include "network_helpers.h"
+
+#ifndef __NR_pidfd_open
+#define __NR_pidfd_open 434
+#endif
+
+static const char * const bash_envp[] = { "TMPDIR=shouldnotbeset", NULL };
+
+static inline int sys_pidfd_open(pid_t pid, unsigned int flags)
+{
+	return syscall(__NR_pidfd_open, pid, flags);
+}
+
+static int update_storage(int map_fd, int secureexec)
+{
+	int task_fd, ret = 0;
+
+	task_fd = sys_pidfd_open(getpid(), 0);
+	if (task_fd < 0)
+		return errno;
+
+	ret = bpf_map_update_elem(map_fd, &task_fd, &secureexec, BPF_NOEXIST);
+	if (ret)
+		ret = errno;
+
+	close(task_fd);
+	return ret;
+}
+
+static int run_set_secureexec(int map_fd, int secureexec)
+{
+	int child_pid, child_status, ret, null_fd;
+
+	child_pid = fork();
+	if (child_pid == 0) {
+		null_fd = open("/dev/null", O_WRONLY);
+		if (null_fd == -1)
+			exit(errno);
+		dup2(null_fd, STDOUT_FILENO);
+		dup2(null_fd, STDERR_FILENO);
+		close(null_fd);
+
+		/* Ensure that all executions from hereon are
+		 * secure by setting a local storage which is read by
+		 * the bprm_creds_for_exec hook and sets bprm->secureexec.
+		 */
+		ret = update_storage(map_fd, secureexec);
+		if (ret)
+			exit(ret);
+
+		/* If the binary is executed with securexec=1, the dynamic
+		 * loader ingores and unsets certain variables like LD_PRELOAD,
+		 * TMPDIR etc. TMPDIR is used here to simplify the example, as
+		 * LD_PRELOAD requires a real .so file.
+		 *
+		 * If the value of TMPDIR is set, the bash command returns 10
+		 * and if the value is unset, it returns 20.
+		 */
+		execle("/bin/bash", "bash", "-c",
+		       "[[ -z \"${TMPDIR}\" ]] || exit 10 && exit 20", NULL,
+		       bash_envp);
+		exit(errno);
+	} else if (child_pid > 0) {
+		waitpid(child_pid, &child_status, 0);
+		ret = WEXITSTATUS(child_status);
+
+		/* If a secureexec occurred, the exit status should be 20 */
+		if (secureexec && ret == 20)
+			return 0;
+
+		/* If normal execution happened, the exit code should be 10 */
+		if (!secureexec && ret == 10)
+			return 0;
+	}
+
+	return -EINVAL;
+}
+
+void test_test_bprm_opts(void)
+{
+	int err, duration = 0;
+	struct bprm_opts *skel = NULL;
+
+	skel = bprm_opts__open_and_load();
+	if (CHECK(!skel, "skel_load", "skeleton failed\n"))
+		goto close_prog;
+
+	err = bprm_opts__attach(skel);
+	if (CHECK(err, "attach", "attach failed: %d\n", err))
+		goto close_prog;
+
+	/* Run the test with the secureexec bit unset */
+	err = run_set_secureexec(bpf_map__fd(skel->maps.secure_exec_task_map),
+				 0 /* secureexec */);
+	if (CHECK(err, "run_set_secureexec:0", "err = %d\n", err))
+		goto close_prog;
+
+	/* Run the test with the secureexec bit set */
+	err = run_set_secureexec(bpf_map__fd(skel->maps.secure_exec_task_map),
+				 1 /* secureexec */);
+	if (CHECK(err, "run_set_secureexec:1", "err = %d\n", err))
+		goto close_prog;
+
+close_prog:
+	bprm_opts__destroy(skel);
+}

tools/testing/selftests/bpf/progs/bprm_opts.c

+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright 2020 Google LLC.
+ */
+
+#include "vmlinux.h"
+#include <errno.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+struct {
+	__uint(type, BPF_MAP_TYPE_TASK_STORAGE);
+	__uint(map_flags, BPF_F_NO_PREALLOC);
+	__type(key, int);
+	__type(value, int);
+} secure_exec_task_map SEC(".maps");
+
+SEC("lsm/bprm_creds_for_exec")
+int BPF_PROG(secure_exec, struct linux_binprm *bprm)
+{
+	int *secureexec;
+
+	secureexec = bpf_task_storage_get(&secure_exec_task_map,
+				   bpf_get_current_task_btf(), 0,
+				   BPF_LOCAL_STORAGE_GET_F_CREATE);
+
+	if (secureexec && *secureexec)
+		bpf_bprm_opts_set(bprm, BPF_F_BPRM_SECUREEXEC);
+
+	return 0;
+}